Clinical application of neutron radiotherapy has expanded rapidly in recent years, but progress in precision neutron dosimetry has been hampered by a lack of basic nuclear physics data. The problem is most severe for those clinical facilities which produce neutron spectra having a substantial fraction of neutrons with energy greater than 20 MeV. In this energy region there is no adequate data base for the calculation of significant dosimetry quantities such as the relative response of "tissue-equivalent" dosimeters. These calculations are required if the delivered dose to tissues of various chemical composition is to be related with confidence to the dosimeter response. Until recently, few laboratories were equipped to perform the required measurements. Ohio University has initiated, and seeks to complete, a series of measurements of cross sections and energy spectra of charged particles produced in the bombardment of carbon, oxygen and nitrogen by 20-26 MeV neutrons. A new spectrometer has been developed and shown to be well suited to this task. Additional areas of research which are of central importance in neutron dosimetry have been identified and proposed: The theoretical problem of dose calculation is sensitive to parameters of the neutron optical model potential which have not been precisely specified in this energy region. Determination of a good parameter set in the energy range from 20-40 MeV is an important goal of this program. A direct test of theoretical calculations by measurng the response of a tissue-equivalent proportional counter for several monoenergetic neutron fields should serve to eliminate an apparent discrepancy between these calculations and recent experimental measurements.